Making a glass rod with a step of measuring an incremental weight gain

ABSTRACT

The present invention provides a simple method for fabricating fiber-optic glass preforms having complex refractive index configurations and/or dopant distributions in a radial direction with a high degree of accuracy and precision. The method teaches bundling together a plurality of glass rods of specific physical, chemical, or optical properties and wherein the rod bundle is fused in a manner that maintains the cross-sectional composition and refractive-index profiles established by the position of the rods.

This application is a division of, and claims priority to, prior U.S.patent application Ser. No. 09/778,329 originally filed Feb. 6, 2001 nowU.S. Pat. No. 6,711,918 entitled “PREFORM FOR PRODUCING AN OPTICAL FIBERAND METHOD THEREFOR”.

The United States Government has rights in this invention pursuant toContract No. DE-AC04-94AL85000 between the United States Department ofEnergy and Sandia Corporation for the operation of Sandia NationalLaboratories.

BACKGROUND OF THE INVENTION

The present invention is a method for fabricating fiber-optic preformswith complex refractive-index and/or dopant distributions to a highdegree of accuracy and precision. In particular, the present inventionfocuses on the fabrication of performs for providing rare-earth-dopedoptical fibers such as those widely used in fiber amplifiers and lasers.

The simplest method of preform fabrication is the so-called“rod-in-tube” method such as is disclosed and described in U.S. Pat.Nos. 4,668,263 and 4,264,347. A rod of glass that will form the core ofthe fiber is inserted into a thick-walled tube that will become thecladding, and the two are fused together at high temperature. Therelative dimensions of the core and cladding in the drawn fiber areidentical to that of the original preform. The main advantage of therod-in-tube technique is its simplicity and as such it was used almostexclusively during the earliest years of fiber development. However,while simple this technique was also quite limited in its ability toimplement optical fiber designs having any but the most rudimentarycharacteristics. Newer methods capable of producing ultra-low-lossfibers, such as are required for optical telecommunications, haveessentially replaced the rod-in-tube technique.

In order to practice the rod-in-tube method, bulk glass is usuallysynthesized by mixing together the various ingredients in powder formand melting the mixture in a high-temperature furnace. All modernpreform fabrication methods, however, are based instead onvapor-position techniques. The core and cladding materials are formed byreacting various gas-phase precursors at high temperature to form aglass “soot” that is subsequently sintered into a solid material. Aprinciple advantage of the vapor-deposition process is its inherentcapacity for providing a built-in purification step that immediatelyprecedes the synthesis step. Starting reagents (liquids or solids) areheated and delivered to a reaction zone as a vapor phase. Thisdistillation-like process leaves behind the vast majority ofcontaminating species typically present as trace constituents in thestarting reagent materials, most notably transition metals.

Three types of vapor-deposition processes have been developed forfabrication of fiber-optic preforms. By far the most widely used methodin the manufacture of rare-earth-doped fibers is the so-called “ModifiedChemical Vapor Deposition” (MCVD) process. In this technique, volatilecompounds, usually halides or chelated complexes, containing the desireddopant species 1, as a gas phase, are reacted with oxygen within aninside portion 2 of a thick-walled silica reaction tube 3, as shown inFIG. 1. As reactants 1 are delivered, silica reaction tube 3 is rotatedwhile its outside surface is heated with an oxygen-hydrogen flame 4. Theflame is translated back and forth along the axis of the tube.Combustion of gas-phase reactants 1 is confined to heated zone 2 a,inside the tube, and deposition of the products of combustion (“soot” 5)occurs on the inner surface 2 a of silica reaction tube 3. Following thecombustion/deposition step, the temperature in the tube is increased to˜1500° C., which sinters the deposited soot 5 into a solid layer ofmaterial. The deposition and sintering cycle is then repeated to buildup additional layers of glass, after which the temperature of the tubeis raised to ≦2000° C., at which point surface tension causes the tubeto slowly collapse inward to form a solid rod serving as the finishedpreform.

In the simplest version of MCVD, silica tube 3 forms the “cladding” ofthe preform (i.e., the region surrounding the core), and vapor-depositedmaterial 5 forms the “core”. One of the main advantages of MCVD,however, is that the chemical composition of the glass can be varied asa function of its radial position in the preform. That is, by adjustingthe mixture of dopant species as each successive layer is deposited, thecomposition of the core and, if desired, of the portion of the claddingformed by the deposition process can be customized for specificapplications. This procedure can thereby be used to achieve a structuredor graded dopant profile in the preform and thus a correspondingstructured or graded refractive-index profile in the subsequentlyfabricated optical fiber.

An important variant of the standard MCVD process is a technique called“solution doping”, which provides an alternative method for introducinga dopant-oxide species into the preform. In this method variation, asoluble salt of one or more dopant species is dissolved in a suitablesolvent, such as alcohol. The partially sintered glass soot is soaked inthe salt solution, and the solvent is subsequently removed byevaporation. The sintering process then proceeds as before,consolidating the dopant species and host material into a solid glasspreform.

Related to MCVD are two other vapor deposition processes, referred to as“Outside Vapor Deposition” (OVD) and “Vapor Axial Deposition” (VAD). Inboth techniques, a chloride of the desired dopant species 1 isintroduced and reacted with H₂O generated in an oxygen/hydrogen flame.Flame 4 is directed against solid substrate 6 where soot 5 is deposited.The substrate in the OVD process is a rotating silica rod, as shown inFIG. 2. When enough material has been deposited, the partially sinteredboule of glass is removed from the silica rod and fully sintered. Thesintered mass is then collapsed, as before, at high temperature to formthe solid glass preform. In the VAD technique, torch flame 4 is directedonto the end of a rotating silica pedestal 7 as shown in FIG. 3. As withMCVD, solution doping can be used with the OVD and VAD processes toincorporate additional dopant species into the pre-sintered glasspreform before the final sintering step is carried out. The maindifferences between the OVD and VAD techniques are:

1) The radial profiles of the dopant species (including rare-earthconstituents and other species such as B, Al, P, Ge, and F), andtherefore the refractive index, can be controlled more easily in the OVDprocess.

2) The VAD process eliminates the sometimes difficult step of removingthe pre-sintered soot boule from the silica rod.

3) The VAD process does not require the preform-collapse step.

A characteristic common to all vapor-deposition techniques is poorprocess control. Delivering known and stable concentrations of dopantprecursor species is particularly difficult. The rare-earth chlorides,for example, must be delivered as vapor through heated delivery lines toavoid recondensation. In addition, these species are very reactive,making it difficult to use mass-flow controllers or similar devices toregulate reactant flow rates and therefore rates of species addition.Furthermore, fluctuations in the temperature distribution of thereaction zone affect the composition of the preform by changing therelative rates of the various oxidation reactions and by changing thesoot deposition efficiency. Similarly, with the solution dopingtechnique, the distribution of dopant species incorporated into the hostmaterial is often non-uniform and unpredictable (the density and poresize of the partially sintered glass network can vary substantially). Inpractice, it is usually necessary to adjust the various processparameters by trial and error, fabricating several preforms until one ofacceptable quality is obtained. Where tolerances on refractive indexand/or dopant concentration are important, or where the shapes of therequired dopant and/or refractive-index profiles are complex, theprobability of producing a preform having an acceptable level of qualitydecreases dramatically. As a result, the range of fiber designs that canbe fabricated is quite limited. This limitation persists despite largeinvestments of time and resources in the development of optical fibersfor a wide variety of commercially significant applications {see S. E.Miller and A. G. Chynoweth eds., Optical Fiber Telecommunications(Academic Press, San Diego, Calif., 1979); P. C. Becker, N. A. Olsson,and J. R. Simpson, Erbium-Doped Fiber Amplifiers (Academic Press, SanDiego, Calif., 1999)}.

The present invention is directed toward solving these problems byproviding a technique wherein a plurality of rods is bundled and fusedinto a glass preform, which is subsequently drawn into an optical fiber.Related art includes the development of multicore optical fibers (U.S.patent Ser. Nos. 6,041,154; 5,706,825; 4,613,205; and 4,011,007), inwhich several cores share a common cladding, e.g., for passiveimage-transfer applications. Although the present technique provides theflexibility to fabricate similar structures (and many others), such“multiple fibers” are not the emphasis of this invention, nor do theyhave the novel properties of the fibers discussed below.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a practical method forfabricating a glass preform to provide drawn optical fibers havinghighly controlled and controllable compositions, both perpendicular to,and parallel with, the drawn glass fiber axis, and therefore providingoptical fibers having highly controlled and controllable physical,chemical, and optical properties.

It is another object of the invention to provide a method for providinga glass preform for use in fabricating an optical fiber having a complexcross sectional structure.

It is yet another object of the invention to provide a method forproviding a glass preform for use in fabricating an optical fiberincorporating internal structures having physical, chemical, and opticalproperties that can be simply and easily contained within a predefined,fixed location.

Still another object of this invention is to provide a method forproviding a glass preform for use in fabricating a single-mode opticalfiber having a large mode-field area.

Yet another object of this invention is to provide a glass preform foruse in fabricating a single-mode or multimode optical fiber with a corenumerical aperture below 0.1.

Another object of this invention is to provide a glass preform for usein fabricating a multimode optical fiber with properties that facilitatesuppression of light propagation in the LP₁₁ and higher-order modes.

Another object of this invention is to provide a glass preform for usein fabricating a multimode optical fiber with properties that providepreferential gain for light propagating in the fundamental mode (LP₀₁).

A further object of this invention is to provide a glass preform for usein fabricating a multimode optical fiber having a non-uniform dopantdistribution within a central core region.

Still another object of the invention is to provide a glass preform foruse in fabricating a polarization-maintaining optical fiber, and forproviding such a fiber exhibiting any or all of the foregoingcharacteristics.

It is still another object of the invention to provide a glass preformfor use in fabricating a double-clad optical fiber and such a fiberwherein an amplified-spontaneous-emission-absorbing dopant isincorporated in an inner clad region of said optical fiber.

Yet another object of the invention is to provide a glass preform forfabricating optical fibers having any combination of the foregoingproperties and characteristics.

The foregoing objects are meant as illustrative of the invention onlyand not as an exhaustive list. These and other objects will becomeapparent to those having ordinary skill in these arts as the inventionis described in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of the Modified Chemical VaporDeposition (MCVD) technique for providing a sintered glass preform.

FIG. 2 shows a variant of the MCVD technique known as Outside VaporDeposition (OVD) wherein the sintered glass preform is formed on theoutside of a substrate rod.

FIG. 3 shows a second variant of the MCVD technique known as Vapor AxialDeposition (VAD) wherein the sintered glass preform is formed on anoutside end of a substrate rod.

FIG. 4 illustrates a preform bundle of the present invention forproviding a step-index optical fiber, wherein a cladding portioncomprises a random distribution of glass rods each having either ahigher or lower refractive index than a target refractive index suchthat the average cladding index equals the target index.

FIG. 5 shows the elemental and refractive-index profiles of arare-earth-doped preform fabricated by the MCVD process and illustratingthe effects of “burnout.”

FIGS. 6A-C show the effect of spatial averaging (bundling a plurality ofdoped rods or fibers) that mitigates the problem of “burnout.”

FIG. 7 illustrates a preform bundle of the present invention forproviding a step-index optical fiber, wherein a cladding portioncomprises a random distribution of glass rods each having either ahigher or lower refractive index than a target refractive index, andadditionally including a central core region containing a confinedrare-earth-doped region.

FIGS. 8A-D shows four designs used in commercially availablepolarization-maintaining optical fibers (for passive lighttransmission).

FIG. 9 illustrates a preform bundle of the present invention forproviding a step-index optical fiber similar to that shown in FIG. 7 andadditionally including two sectors confined within opposing segments ofthe cladding region which contain rods having thermal expansionproperties designed to impart an internal stress to a finished opticalfiber.

FIG. 10 illustrates a simple assembly technique for preparing a rodbundle.

FIG. 11 illustrates the assembled rod bundle contained in a silica glasstube suitable for cleaning and drying of the bundle.

FIG. 12 shows the finished and sealed glass ampule containing the rodbundle of the present invention.

FIG. 13 illustrates a schematic of the furnace apparatus for processingthe sealed preform ampule.

FIG. 14 illustrates how the reactant gas-delivery system of aconventional MCVD setup would be modified to provide a method forpreparing the glass soot as a powder.

FIG. 15 illustrates how the collection vessel used to collect the glasssoot of the MCVD process as a powder is modified to serve as a cruciblefor melting the collected powder and then drawing the glass melt as arod or fiber.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 is exemplary of the present invention, wherein preform 40 isfabricated from a large number of glass rods bundled together and thenfused at high temperature. FIG. 4 shows such a preform, looking down theaxis of the preform, prior to fusion. The preform shown in this exampleis for a rare-earth-doped fiber with a stepped refractive index core 42of uniform dopant density (a “step-index” profile).

Core region 42 of the preform contains rods 42 a fabricated fromrare-earth-doped glass with a refractive index of n_(core). Claddingregion 41 of the preform contains two different types of glass rods, 41a and 41 b, neither of which contains rare-earth dopants. One type ofcladding rod has a refractive index slightly greater than the desiredcladding refractive index, n_(clad). The other type of cladding rod hasa refractive index slightly less than n_(clad). The ratio oflow-index/high-index cladding rods is chosen so that the average indexof refraction in the cladding region is equal to n_(clad); in theexample shown in FIG. 4, the placement of low-index/high-index claddingrods is random. This bundle is then fused to make a solid preform. Aswith conventionally fabricated preforms, the relative sizes of the coreand cladding in the preform will be retained in the fiber.

When the preform is drawn into fiber, the “granularity” of therefractive-index distribution in the cladding region will be largelypreserved. If this granularity is made fine enough, however, such afiber will behave as though the cladding were made of a single,homogeneous material with index n_(clad). Small-scale variations in theindex of refraction are, in effect, averaged over by the mode field ofthe light propagating in the fiber. Similarly, it is clear that the coreof the fiber is not truly circular in shape. The circular region in thecenter of FIG. 4 is made up of a finite number of pixels (rods).However, the mode-field distribution for light traveling down the fiberis known to be insensitive to the fine-scale features of therefractive-index distribution. If the number of pixels (number of rodsin the bundle) is made large enough, the mode “sees” a circular core.

Fortunately, the number of pixels required to obtain the desiredrefractive-index-averaging effect is not prohibitively large. Thepresent invention therefore allows the properties of the preform to beengineered to almost arbitrary specifications. For example, to changethe numerical aperture NA; where:

NA=(n _(core) ² −n _(clad) ²)^(1/2)

of the preform shown in FIG. 4, one would simply change the ratio oflow-index to high-index rods in the cladding region to change n_(clad).Such adjustments can be made with excellent accuracy (i.e., control ofthe absolute refractive-index value) and precision (reproducibility). Inaddition, this technique is very versatile and practical in itsimplementation in that a wide variety of preforms can be fabricatedusing only two different types of cladding rods.

Representative Applications to Fiber Lasers and Amplifiers

The utility of the present invention is best illustrated by example. Inthe following sections several potential applications to fiber lasersand amplifiers that are of current interest are outlined.

Large-mode-area, Single-mode Fibers

One area of considerable recent activity is the development ofsingle-mode fibers with large mode-field areas. Such fibers are ofinterest for lasers and amplifiers capable of generating very high peakpower pulses and for narrow-linewidth fiber sources capable ofgenerating high average powers. One known approach to increasing themode-field area while preserving single-mode operation (required tomaintain diffraction-limited beam quality) is to lower the NA of thefiber. The NA for a typical telecommunications fiber is in the range of0.15 to 0.20. For pulsed fiber amplifiers, NA's of 0.1 and lower are ofinterest, with the lower limit ultimately determined by fiber bend-lossconsiderations. The fabrication of preforms for ultra-low NA fibers,however, is neither straightforward nor trivial.

The refractive-index difference (Δn) between the fiber core and claddingregions for conventional telecommunications fibers is typically 0.01 to0.02. In comparison, the Δn value for a 0.05 NA step-index fiber issmaller by a factor of 10 to 20. With conventional preform-fabricationtechniques it is very difficult to achieve the level of precision andaccuracy in the refractive-index distribution required for ultra-low-NAfibers. Furthermore, poor accuracy in the refractive-index distributionwill result in fibers that have too large an NA, or in fibers that areweakly or altogether non-guiding. Even if the average Δn in the preformis very close to the target value, poor precision results in variationsin NA along the length of the fiber that greatly increase sensitivity tobend loss. These considerations are of special concern forrare-earth-doped fibers since the fabrication process typically utilizesa multiplicity of dopant species, all of which must be carefully andsimultaneously controlled. Typical dopants include one or morerare-earth-ions taken from the Lanthanide Series of elements, as well asrefractive-index raising/lowering dopants and dopants used to enhancethe solubility of the rare-earth ions (e.g., compounds containingspecies taken from elements on the Periodic Table of Elements designatedas new IUPAC Groups 13-17, such as boron, aluminum, silicon,phosphorous, and germanium, certain members of the Halide Group, e.g.fluorine, and various members of the Transition metals listed in newIUPAC Groups 3-12, such as zirconium, titanium, and zinc).

This situation is further complicated by the need to fabricaterare-earth-doped fibers intended for high-peak-power operation. Fibersof this type require as high a rare-earth-dopant density as possible.However, this requirement conflicts with the low-NA requirement becausehigh dopant densities in the core typically leads to large Δn since, asdiscussed earlier, in the widely practiced MCVD technique, the claddingglass is usually undoped silica whose index of refraction issubstantially less than that of the rare-earth-doped glass.

The present invention allows the problems of poor process control andincompatibility between core composition and Δn to be circumvented.Referring back to FIG. 4, the requirement for high rare-earthconcentration in the core can be met by fabricating core rods with ashigh a dopant density as possible, without concern for the refractiveindex. The cladding is then constructed using the appropriate mixture ofhigh- and low-index cladding rods to achieve the desired target Δn. Theproblem of refractive-index incompatibility is eliminated because therefractive index of the cladding can be tailored to that of the core.Furthermore, because the cladding refractive index is very wellcontrolled, the requirement for a small (and uniform) core/claddingindex difference does not present a problem.

Multimode Fibers

As noted earlier, there is a limit to how low the NA can be made in apractical fiber amplifier. Further increases in mode-field area can berealized by using a multimode gain fiber that is constrained to operateon only the lowest-order transverse mode (LP₀₁). One way to obtain suchsingle-mode operation in a multimode amplifier is to carefully controlthe launch conditions of the signal being amplified; the signal injectedinto the multimode amplifier should ideally excite only the LP₀₁ mode.Another technique that can be used to obtain preferential amplificationof signals in the LP₀₁ mode is to use bend loss to discriminate againsthigher-order modes. In both approaches, the second lowest order mode(LP₁₁) is the most difficult to suppress.

Conventional preform fabrication techniques (with the exception of VAD)entail a final step in which the cladding tube, with an inner coating ofmaterial formed during the vapor-deposition process, is collapsed toform a solid rod (the preform). The highest temperatures are reachedduring this step in the fabrication process, and it is at these elevatedtemperatures that a phenomenon known as “burnout” occurs, wherein someof the co-dopant species, most notably Ge and P compounds, undergothermal decomposition. Thermal decomposition occurs preferentially atthe inner surface of the preform (which will become the central regionof the core following preform collapse), where gas-phase products areable to escape as they are evolved. The effects of burnout in thefinished preform are shown graphically in FIG. 5 where the concentrationof dopant species and the refractive index of the affected preform ismeasured across a diameter of the preform. As seen in FIG. 5, in thecentral or core region of the preform, the refractive index is reducedand the rare-earth-dopant concentration is substantially depleted,resulting in donut-shaped refractive-index and dopant distributions.This characteristic is preserved when the preform is drawn into a fiberand is ubiquitous in preforms fabricated by either the MCVD or the OVDprocess.

Because the LP₁₁ mode also has a donut-shaped intensity distribution, itis heavily favored over the LP₀₁ mode in a multimode fiber that hassustained the effects of burnout because: i) the donut-shapedrefractive-index profile makes it difficult to propagate light in theLP₀₁ mode since light injected into the central portion of the core isinstead guided into the higher-refractive-index annular region at theperimeter; and ii) the small-signal gain depends exponentially on theoverlap integral of the dopant and mode-field distributions. Theintensity maximum of the LP₀₁ mode coincides with the “hole” in thedopant distribution. Conversely, the LP₁₁ mode and the donut-shapeddopant distribution are well matched to each other. For these reasons,burnout results in refractive-index and dopant profiles that are exactlythe opposite of what is required for operation on the lowest-order mode.

In the present invention, the problems associated with preform burnoutcan be eliminated, for the following reasons:

1) Because adjustments to the core and cladding indices are decoupled,the core glass can be fabricated using co-dopants that are not subjectto thermal decomposition at high temperature without any constraintsrelated to fiber NA.

2) To the extent that thermal decomposition does occur duringhigh-temperature processing, the shape of the refractive-index anddopant profiles are not altered in a manner that disfavors LP₀₁, asshown in FIG. 6.

3) Finally, as will be described later, the forces on the preform bundleduring the final fusion step are considerably greater than theaforementioned surface tension relied upon in the MCVD process and assuch the temperature required for preform bundle fusion can be made lowenough to prevent thermal decomposition from occurring.

The goal, therefore, of achieving a true step-index profile and asimilar dopant profile, or a variety of other profiles described below,can be realized.

Fibers with Non-uniform Dopant Distributions

The above discussion of burnout suggests how the design of a multimodefiber laser/amplifier might be further improved to favor amplificationof the lowest-order mode. Because the present invention allows directcontrol over the refractive-index and dopant distributions, morecomplicated preform designs intended to optimize discrimination betweenthe LP₀₁ and LP₁₁ modes are feasible. The simplest form of suchoptimization would be to restrict the rare-earth dopant to the centralportion of the core since in this embodiment amplification coincideswith the intensity maximum of the LP₀₁ mode in the central region of thecore and with the intensity minimum of the LP₁₁ mode.

With the present invention, the design and fabrication of suchcustomized preforms becomes realistic. FIG. 7 shows a representativepreform for a step-index fiber with a cladding-to-core diameter ratio ofabout 10:3 and wherein the rare-earth dopant is confined to a centralregion of the core having a diameter about one-half (½) that of the coreregion. (Typical, representative dimensions of these regions would be a200 μmØ padding and a 60 μm Ø core region, comprising a 15 μm thickannular ring surrounding a 30 μm Ø central, rare-earth-doped core zone.Each of these dimensions may be varied, however, to suit therequirements of the application.)

In the annular, undoped region of the core (the “core annulus”), of FIG.7, the ratio of high/low refractive index rods is adjusted to match therefractive index of the rare-earth-doped rods. As shown in FIG. 7,placement of the high/low refractive index rods is random, or nearly so.

It is likely that even better suppression of the LP₁₁ mode could beobtained with more complicated dopant and for refractive-indexdistributions (e.g., radially graded profiles, with therare-earth-dopant concentration and/or the refractive index decreasingmonotonically with distance from the center of the core). The presentinvention makes such preform designs straightforward to implement in asystematic and controlled manner.

Polarization-maintaining Fiber

In many applications, the output polarization state of a fiberlaser/amplifier is important. Because of fiber birefringence, the outputpolarization of conventional rare-earth-doped fiber amplifiers is ingeneral elliptical and time-varying. The best solution to the problem offiber birefringence is the use of Polarization Maintaining (PM) fiber.In a PM fiber, the propagation constants (indices of refraction) aremade sufficiently different for two orthogonal axes (e.g., horizontaland vertical) that light polarized along one axis is not stronglycoupled to the other axis. Linearly polarized light launched along oneof the polarization axes of a PM fiber therefore remains linearlypolarized, with negligible power transferred to the other polarizationstate. One way to make the indices of refraction different for the twoorthogonal linear polarization states is to place the fiber in a stressfield that is cylindrically asymmetric. The most common approach togenerating the required stress field is the incorporation of stressmembers into the cladding of the preform. The stress members are madefrom a glass whose coefficient of thermal expansion is substantiallydifferent (usually larger) than that of the cladding glass, resulting ina stress field that is permanently frozen into the fiber oncefabricated.

FIG. 8 shows the various designs for stress elements used incommercially available PM fibers. Note that none of the PM fibers shownin FIG. 8 contain any rare-earth dopant (i.e., they are used for passivetransmission of polarized light, but not for amplification). There iscurrently only one rare-earth-doped PM fiber commercially available, asingle-clad Er-doped fiber manufactured by FiberCore in the UK.Double-clad, rare-earth-doped, PM fibers have recently been reported,but they are not widely available. (Note: in a single-clad,rare-earth-doped fiber, the pump and signal beams are confined to thecore of the fiber. In a double-clad fiber, the cladding region isconverted into a high-NA multimode waveguide, referred to as the “innercladding,” by adding a low-index polymer coating to the outside of thefiber. The advantage of a double-clad fiber is that much larger pumppowers can be coupled into the fiber using multimode pump sources bylaunching the pump light into the inner cladding rather than into thecore. The pump light is still absorbed in the core, and the signal lightstill propagates in the core.)

The fabrication of a cylindrically asymmetric structure is difficultusing traditional methods for preform manufacture. In contrast, in thepresent invention, the incorporation of stress rods is straightforward.FIG. 9 depicts a PM version of the preform shown in FIG. 7. As in aconventional (passive) PM fiber, the material used for the stress rods(e.g., borosilicate) would have a coefficient of thermal expansionsubstantially different from that of the cladding glass.

By fabricating the stress rods from a glass whose index of refraction isless than that of the cladding glass, the problem of helical rays (raysthat are confined to the inner cladding but do not intersect the core ofthe fiber) can be eliminated. In a conventional double-clad fiber, thetrajectories of helical rays are scrambled by making the cross-sectionof the inner cladding non-circular (e.g., a rectangle or hexagon).Alternatively, the problem of helical rays can be circumvented byoff-setting the core from the center of the inner cladding. In bothcases, the preform must be carefully ground and polished, and possiblyre-sleeved, to achieve the desired shape before drawing. With thepresent invention, it is straightforward to construct a preform in whichthe stress rods provide the required mode scrambling effect. Thisapproach makes it possible to use a preform of circular cross-section;in addition to simplifying the preform fabrication process, adouble-clad fiber of circular cross-section is advantageous from thestandpoint of fiber cleaving and fusion splicing. Furthermore, forapplications in which an off-set core or a non-circular inner claddingis desirable, the present invention allows fabrication of the requiredpreform without machining or re-sleeving (see below).

Double-clad Fibers with Very High-NA Inner Cladding

As mentioned above, the advantage of a double-clad (cladding-pumped)fiber is that much more pump light can be launched into the fiber (atmuch lower cost) than with a single-clad (core-pumped) fiber. Thisadvantage results from two effects: 1) the cross-sectional area of thecladding is much larger than that of the core, and 2) the inputacceptance angle is much greater for the high-NA inner cladding than forthe lower-NA core. The NA of the inner cladding is determined by thedifference in refractive index between the low-index polymer coating andthe silica cladding glass. An NA of 0.35 is obtained with a siliconecoating, and NA's as high as 0.47 can be achieved with more recentlydeveloped fluoropolymers. As described earlier, the present inventionmakes it possible to use cladding materials other than pure silica. As aresult, the NA of the inner cladding can be increased significantly byincreasing n_(clad). For example, one material that is promising for theconstruction of rare-earth-doped fibers is a mixed alkali-zinc-silicateglass manufactured by Schott Glass Technologies Inc., and identified asIOG-10. The index of refraction of IOG-10 is 1.530, allowing the NA offluoropolymer-clad fibers to be increased from 0.47 to 0.66. This NAcorresponds to greater than a two-fold increase in the amount of pumplight that can be coupled into the double-clad fiber, for a pump sourceof a given brightness.

Double-clad Fibers with ASE-absorbing Dopants in the Inner Cladding

In any fiber amplifier, an upper limit to the population inversion(i.e., to the stored energy and the gain) is determined by a processknown as “Amplified Spontaneous Emission” (ASE). Although most ASEpropagates in the core, in a double-clad fiber, a significant amount ofpower can be lost to ASE propagating in the high-NA inner cladding. Inaddition to reducing the population inversion, cladding ASE can degradethe output beam quality and can cause parasitic “lasing” when the gainis not lowered by another process (e.g., by seeding the amplifier withsufficient power). Approaches to reducing cladding ASE includingangle-polishing the fiber (although very large angles are required tosuppress lasing in the high-NA inner cladding) and mode-stripping theends of the fiber (if the fiber is end-pumped, only one end can bemode-stripped). Both these techniques can only suppress cladding ASE atthe ends of the fiber: they allow ASE to propagate in the innercladding, but they prevent it from emerging from the amplifier or frombeing recirculated by back-reflections from the fiber ends.

A superior approach for suppressing cladding ASE would be to dope theinner cladding with a material that absorbs ASE but does not absorb pumplight (which is to be absorbed only in the core). For example, whereerbium has been used as a core dopant, the rare-earth metal terbiumcould serve as the ASE-absorbing species. This approach would have theadvantage of providing distributed suppression of cladding ASE, i.e., itwould prevent ASE from experiencing gain along the entire fiber. Thisapproach has not been employed in double-clad fibers fabricated byconventional techniques, perhaps because of the danger of introducing acontaminant into the MCVD apparatus that, if present in the fiber core,would cause unacceptably high losses for the signal beam. This risk iseliminated by the present invention, in which the cladding rods can befabricated in a different apparatus than are the core rods, ensuringthat contamination of the core will not occur.

The ASE-absorbing dopant would likely be contained in only part of theinner cladding (e.g., in a ring well outside the core). In a single-modefiber, the electric field of the light propagating in the core hassignificant amplitude in the cladding; should this field interact withthe ASE-absorbing dopant, the fiber would experience excessive signalloss. Restricting the ASE-absorbing dopant to the outer portion of theinner cladding would minimize or eliminate this loss. (In the multimodefibers discussed above, less of the core light propagates in thecladding, reducing the importance of this consideration.) Anotheradvantage of placing the ASE-absorbing dopant in the outer portion ofthe inner cladding is that the refractive index of these rods would nothave to be well-matched to that of the cladding; the core NA will dependonly on the refractive indices of the core rods and the regular claddingrods adjacent to the core (i.e., those not doped to absorb ASE).Moreover, because the core light does not interact strongly with theASE-absorbing rods, they can be relatively lossy and can be fabricatedusing standard, bulk-glass techniques (ultra-high purity is notrequired).

Double-clad Fiber with an Off-set Core or a Non-circular Inner Cladding

As mentioned above in the context of PM fibers, two approaches tocircumventing the problem of helical rays in double-clad fibers (i.e.,rays propagating in the inner cladding that do not intersect the fibercore) are: (1) to off-set the core from the center of the fiber,typically by grinding and possibly re-sleeving of the preform; and (2)to make the inner cladding non-circular. The present invention allowspreforms with either or both of these features to be fabricateddirectly. For achieving an off-set core, the core rods would be locatednon-centrally in the bundle. For obtaining a non-circular innercladding, the outer tube used during the construction of the bundlewould have the desired non-circular shape; alternatively, the bundlewould be cylindrical, but it would include etchable glass rods thatwould provide the desired non-circular shape after etching. Of course,these features can be combined with any of the other features discussedabove (i.e., non-uniform dopant distribution, low NA, etc.).

BEST MODE FOR IMPLEMENTING THE INVENTION

In the following sections, methods are described that may be used toimplement the present invention for the fabrication of fiber preforms.This description is not meant to be exhaustive; rather, it outlines someof the considerations involved in reducing the invention to practice,and it demonstrates that the invention is practical for the fabricationof useful preforms with unique and hitherto unattainablecharacteristics.

The discussion will focus on silica-based fibers, which are by far themost common. This emphasis, however, does not and therefore should notbe interpreted to imply that the invention is applicable only tosilica-based fibers. It is, in fact, applicable to a wide variety ofglass compositions, including halide-based glasses (e.g., fluoride or“ZBLAN” glass), chalcagonide glasses (e.g. sulfide, selenide, andtelluride glasses), and various multi-component glasses (e.g.,SiO₂—Al₂O₃—NaO₂—CaO) comprising compounds of boron, silicon, aluminum,phosphorous, germanium, zinc, titanium, zirconium, any of the alkali andalkaline-earth elements and/or any of the various alloys thereof. Theterm “glass,” therefore, is intended by the Applicants to be interpretedbroadly to mean any material that is or has been found to have utilityas an optical fiber that is comprised of and prepared from the abovelist of materials.

Similarly, the discussion will focus on fibers with a circular claddingand with a circular core located in the center of the cladding. Preformbundles of the present invention, as seen in FIGS. 4 and 7-9, take on agenerally cylindrical shape when assembled, especially as the size ofthe underlying rods decreases and their numbers greatly increase. Thisemphasis, however, does not and therefore should not be interpreted toimply that the invention is applicable only to circular fibers withcircular, centrally located cores. It is, in fact, applicable to anycladding with a closed cross-sectional shape (including elliptical,square, rectangular, hexagonal, octagonal, and rhombic), to core bundleswith a comparable variety of shapes, and to any core position. All thatis required is that the preform bundle be contiguous and constrained.

Assembly of Preform Bundles

In each of the preform bundles described thus far, random placement ofthe high/low-index rods in the cladding and core-annulus regions hasbeen assumed. In such a scheme, the low/high index rods could be counted(individually or by weight) and mixed together thoroughly before beingincorporated into the preform bundle. If the number of rods in thepreform bundle is large, the possibility of obtaining an “uneven”refractive-index distribution that exerts any significant effect on themode-field distribution is remote. For bundles consisting of a smallernumber of rods, semi-random or non-random placement of rods are bothoptions. Semi-random placement largely preserves the main advantage ofrandom placement, i.e., there is no need to place each rod individually.In this approach, the bundle is constructed using random placement, butthe low/high-index rods are color coded or otherwise marked so that theycan be identified when viewed end-on; any “clumps” of high/low-indexrods that result from poor mixing or statistical variation can then bevisually identified and redistributed if necessary. Non-random placemententails the distribution of rods in a predetermined and regular pattern,most likely by an automated device.

A different approach is to fabricate the cladding from a collection ofidentical rods: rods composed of a composite material whose averagerefractive index is equal to n_(clad). These composite cladding rodswould themselves be fabricated from a preform bundle containing amixture of high/low-index rods in the appropriate ratio. In thistwo-step process, the effective pixel density in the cladding of thefinished preform would be equal to the product of the pixel densitiesfor each step. The length scale for random variations in refractiveindex would therefore be constrained to be less than or equal to thediameter of the composite cladding rods.

FIG. 10 shows how the assembly process might be accomplished forpreparing a preform bundle 1000. The preform bundle shown in thisexample is for a rare-earth-doped fiber with a step-index core ofuniform dopant density, similar to that of FIG. 4. In a first step, theentire cladding tube 1001 (a thin-walled tube whose purpose is tocontain the bundle) is packed with a mixture of low-refractive-indexrods 1002 a and high-refractive-index rods 1002 b in the appropriateratio. In a next step, cladding rods from the middle portion 1003 of thebundle are removed and replaced with a corresponding volume of core rods(not shown).

As shown in FIG. 10, a preform template 1004 delineates thecore/cladding boundary, showing directly which cladding rods should beremoved. Preform template 1004 can, of course, be modified to improvethe ease with which the transfer of rods is accomplished. In particular,the “stepped” central portion of the template can be replaced with aremovable plug 1005 that allows the user to partially displace thedesired rods, as shown. Plug 1005 then would be removed, and thedisplaced volume in the preform bundle would be back-filled through thehole left behind by the plug with new glass rods having the desiredproperty (e.g., core rods). This procedure, therefore, prevents thecladding rods from inadvertently moving during the replacement processbecause the core region of the preform always contains substantially thesame volume of glass rods as core rods 1003 are displaced.

Finally, those skilled in the art will appreciate that preform template1004 can comprise any number of distinct regions, or plugs, having avariety of shapes, sizes, and locations (e.g., for the stress elementsdescribed in the context of PM fibers). This approach thus provides asimple method for assembling a preform bundle, with wide flexibility inthe range and complexity of physical structures and chemical propertiesimparted to the finished preform.

Consolidation of Preform Bundles

FIG. 11 shows the next stage of processing. Bundle 1000 is transferredinto a second cladding tube 1100 in which it is suspended andimmobilized between two plugs, e.g., of fiberglass wool 1105 (ultra-highpurity silica, available commercially). Fiberglass packing 1105 preventsthe bundle from sliding in cladding tube 1100 and ensures that there isno relative movement of rods within bundle 1000. This second claddingtube 1100 is fabricated with an innerlip or waist 1101 (formed bypartial collapse of the cladding tube under vacuum) to providemechanical support of the above assembly. Because the fiberglass plug isporous, the entire assembly can be cleaned and dried in place, withoutany need to handle the bundle directly, thereby preventingcontamination. The cleaning and drying steps would likely involve bothliquid-phase and gas-phase processes, similar to those used with theMCVD method. The cleaned and dried assembly is then evacuated and thecladding tube sealed off at both ends to form an ampule 1200 as shown inFIG. 12.

The cladding tube can be fabricated from either of the materials usedfor the cladding rods. Alternatively, if hydrofluoric acid is used toremove the cladding tube from the finished preform, any glass withsimilar thermal properties can be used.

FIG. 13 shows how the evacuated ampule is processed at high temperature(typically <2000° C., with the exact value depending on the glasscomposition) to yield the finished preform. The apparatus shown in FIG.13 is designed to fuse the bundle while preventing the formation oftrapped bubbles. The rod bundle does not “melt” in the usual sense butinstead softens substantially, enough so as to fuse the bundle into amonolithic preform. By gradually inserting the ampule into the heatedzone of a tube furnace 1301, fusion begins at one end of the bundle andprogresses slowly, “zipping up” the bundle so that gas bubbles areexcluded. Alternatively, a “ring-burner” system (not shown) could besubstituted as a means for fusing the ampule to form the preform. Theampule is processed inside silica tube 1302 that is carefully cleanedbefore insertion of the ampule. The silica tube 1302 (which does notsubstantially soften when inserted into the furnace/ring-burner) ismounted on a stepper-motor-driven translation stage that controls therate at which the ampule is fed into the furnace/ring-burner. Inaddition, silica tube 1302 is continuously rotated, which preventsslumping of the softened glass and adhesion of the ampule to the innersurface of tube 1302. Order-of-magnitude estimates for the translationrate 1303 and rotation rate 1304 are 1 inch per hour and 20 revolutionsper minute, respectively. As shown in FIG. 13, the entire assembly ismounted at a slight angle 1305 to ensure that the rolling ampule doesnot wander along the axis 1306 of silica tube 1302. Annealing of thepreform occurs as it gradually exits the heated zone. If necessary, thefinished preform can then be treated with hydrofluoric acid to removeany surface contamination from the inner surface of silica tube 1302.

In addition to maintaining a controlled environment in whichcontamination of the bundle (and the inside of the cladding tube) issubstantially eliminated, the evacuated ampule serves another importantfunction. The one-atmosphere pressure differential between the insideand outside of the ampule greatly accelerates the collapse/fusionprocess when the ampule is softened at high temperature. In the MCVD andOVD processes, the force responsible for collapse of the cladding tubeis surface tension. The collapsing force exerted on an evacuated tube isseveral hundred times larger than the force generated by surfacetension. For this reason, the temperature required for the preformcollapse step can be lowered by about 500° C. This large reduction intemperature makes processing of the preform more straightforward andsubstantially reduces or eliminates the problem of dopant burnout. Inaddition, the furnace or ring-burner could be placed in a chamber thatis pressurized to more than one atmosphere, which would provide an evengreater collapsing force on the ampule.

Fabrication of Core and Cladding Rods

The materials required for the core and cladding rods can be synthesizedin powder form using a conventional MCVD setup. In this approach, thesintering process is omitted and as much of the soot as possible iscollected. During a typical MCVD fabrication run, only a fraction of thesoot that is generated in the reaction zone is deposited on the innerwall of the tubing. Most of the soot remains suspended in the exhaustgas and is typically discarded. The transport process that governs thedeposition efficiency is thermophoresis. In thermophoresis, suspendedparticles are transported down a temperature gradient because momentumtransfer from colliding gas molecules is unequal on the “hot” and “cold”sides of the particle. In the reaction zone, the radial temperaturegradient is such that particles generated are transported away from thewalls of the tube into the center of the flow. Further down the tube,the direction of this gradient reverses as a result of cooling of thetube by ambient air. Under these conditions, thermophoresis causesparticles to migrate towards the wall of the tube, where depositionoccurs. A number of techniques have been suggested for improving thedeposition efficiency.

The apparatus 1400, shown in FIG. 14, was designed with theseconsiderations in mind. Reactant vapor stream 1401 is generated by aconventional MCVD gas-delivery system. A small tube furnace 1403 is usedto heat a reaction zone 1404. Again, a “ring-burner” could besubstituted as a means for heating the reaction zone. The flow path andtemperature distribution are such that the efficiency of the depositioninto collection tube 1402 is maximized. Collection tube 1402 isfabricated from high-purity fused silica (such glassware is commerciallyavailable and is used routinely in the semiconductor industry forsynthesis of ultra-pure starting materials). Because final product 1405is stored in the collection tube, there is no need for any furtherhandling. In this way, the complexity of a full-blown MCVD setup (glasslathe, rotary seals, H₂/O₂ torch, motorized translation stage, etc.) isavoided.

Such a process could greatly ease the control requirements necessary toassure that proper compositional ranges are maintained duringconventional MCVD fabrication. As noted earlier, vapor-depositiontechniques are difficult to control. Many require delivery of multiplespecies by vapor transpiration techniques: the rare-earth chlorides, forexample, must be delivered through heated delivery lines to avoidrecondensation. Furthermore, these species tend to be chemicallyaggressive and use of flow regulating devices to control rates ofspecies addition to the reaction zone is problematic due to thepotential for equipment failure. Finally, temperature fluctuations inthe reaction zone effect the composition of the final product bychanging the relative rates of the various oxidation reactions and bychanging the soot deposition efficiency.

However, by simply collecting the oxide soots of individual reactantspecies generated in separate reaction processes in the glass ampule byweight, it is far more likely that a final target glass composition canbe achieved accurately and reproducibly. This result would be achievedby combusting a single reactant gas stream and determining theincremental weight gain of the ampule as the oxide soot collects on itsinterior walls until a target weight is achieved. The process would berepeated with each subsequent reactant specie until each had beencombusted and the desired quantity of its oxide collected. The collectedpowders would be mixed (e.g. by tumbling them within the ampule), andthe ampule would be sealed.

The glass in powdered form is then zone sintered (similar to theprocedure used with OVD and VAD soot preforms) and drawn into rod orfiber using a single-crucible method. The need for a separate cruciblecan be eliminated by incorporating a “break-off” fixture 1501 at thebase of the collection tube, similar to a conventional glass ampule (seeFIG. 15), which provides a hole through which the rods can be drawn.Furthermore, rather that actually breaking off this “tail”, theextension may be used as a self-contained appendage which would begrasped by the fiber drawing mechanism in order to initiate the drawingprocess.

This approach greatly simplifies the fabrication of high purity rods andfurther reduces the possibility of contamination. Zone sintering iscarried out at the beginning of the fiber draw.

Measurement of the Refractive Index

Once the core and cladding rods have been fabricated, a precisemeasurement of the refractive index must be performed. As discussedearlier, the difference in refractive index between the core andcladding rods can be extremely small, and a precise measurement of thesedifferences is required (although an accurate, absolute refractive-indexmeasurement is not necessary). The following procedure provides therequisite precision. A representative rod from each group (e.g., a corerod, a high-index cladding rod, and a low-index cladding rod) is bentinto the shape of a “U”, and the bottom of each “U” is immersed in atemperature-controlled bath of refractive-index-matching fluid. Each rodis placed between a light source and a detector (i.e., light is launchedinto each rod at one end and is detected at the other end). Therefractive index of the fluid can be precisely and reproducibly adjustedby changing the temperature of the bath. As the temperature isincreased, the refractive index of the fluid decreases. As therefractive index of the fluid approaches that of a given rod, thetransmitted power drops abruptly. In a plot of transmitted power vs.temperature, a v-shaped notch is observed, with a minimum at thetemperature corresponding to a perfect refractive-index match. Byrecording the refractive-index-match temperature for each rod, therefractive-index difference between the various rods can be calculated,provided the temperature coefficient (dn/dT) for therefractive-index-matching fluid is known. For mostrefractive-index-matching fluids, dn/dT is approximately 450 ppm/° C.(where ppm denotes “parts per million”), and the precise value of dn/dTcan be measured with a standard refractometer. The temperaturecoefficient for the refractive index of silica is 18 ppm/° C. and canthus be ignored. For an ultra-low NA fiber (NA=0.05), therefractive-index difference between the core and cladding is ˜600 ppm.The temperature of the bath can easily be measured to within ±0.1° C.,which corresponds to an refractive-index uncertainty of ±45 ppm. One mayconclude, therefore, that the proposed refractive-index measurement willhave the required high degree of precision necessary to determine thedifferences among the various rods and thus between the core and cladregions of the fiber.

What is claimed is:
 1. A method for providing a high purity glass rod,comprising the steps of: providing one or more reactant materials;separately reacting each of said one or more reactant materials byseparately heating each of said one or more reactant materials in thepresence of oxygen contained in a flowing gas stream to provide one ormore oxides; collecting said one or more oxides as dispersed powders ina silica ampule, wherein the step of collecting includes collecting apredetermined quantity of said one or more oxide powders by measuring anincremental weight gain of said silica ampule as said one or more oxidepowders are collected; melting said one or more collected oxide powdersand said silica ampule to form a substantially uniform glass boule; anddrawing said boule into said glass rods.
 2. The method of claim 1,wherein said one or more reactant materials are selected from the listconsisting of halide compounds, chelated complexes, and combinationsthereof.
 3. The method of claim 2, wherein said halide compounds arecomprised of elements selected from the list consisting of boron,aluminum, silicon, phosphorous, sulfur, germanium, selenium, tellurium,iron, zinc, zirconium, titanium, and any of the lanthanide rare earthelements.
 4. The method of claim 2, wherein said chelated complexes arecomprised of elements selected from the list consisting of boron,aluminum, silicon, phosphorous, sulfur, germanium, selenium, tellurium,iron, zinc, zirconium, titanium, and any of the lanthanide rare earthelements.